Chemical indicator



Patented Feb. 25, 1947 No Drawing.

. 4 p 1 f This invention relates to the provision of chemical indicator's which, by virtue of inherent color changes, provide a' means for visually identifying the equivalence point of a neutralization reaction. 'Ihe invention findsparticular utility in titrimetric analysis for determining the alkalinity of water, especially boiler feed water, and other solutions. Y

The indicator used in alkalinity analysis should show a "sharp color change at the equivalence point, i.ie; t e point at which the neutralization reaction iscomplete, with no excess of either acid or alkali. 1Forw'ater analysis, this true stoichiometri j endi point has been determined to lie betweena pn'cf about 4.6 and 4.9, depending on solution concentration and temperature.

" Heretofore;practically all volumetric tests for determining'the alkalinity of solutions, and particularly of waters; were conducted with the aid of methyl orange as the indicator. However, the maximum color transition of methyl orange occurs at pH values ranging from about 3.8 to 4.3, which makes the indicator difiicult to use in detecting an end-'pointlying in the pH range 4.6 to 4.9, even inthe hands of. an experienced oper- CHEMICAL INDICATOR Harry 'Fleisher, Greenbelt, Md.

Application A ugust 12, 1944 f SeriaLNo. 545,251

(Granted under the act mi s 3, -18il3,- ias:

amended April 30, 1928;. 3700.6. 757) coloring agentsin proper ratio-to insure maximum accuracy andsensitivity. :The sodium salt of dimethylaminoazobenzene-oecarboxylic acid is in itself a water soluble acid-base indicator, showing color .transitions of, .yellow to red1in passing from the alkaline to the acid. state. However, this change-is rather difficult to'detect and'furthermore occurs at a pH valuenot within the limits reviously;establishedv forthe neutralization reaction, 1. ,e.,"4.6 to 4.9. +1 have discovered that achromatic effects induced by'the combination ofdissimilar colors make the indicator color transition. more. readily vdetectable, and further;

, cause the-saidtransition to'occur within the-pH ator. For this reason, numerous attempts have V been -made by various investigatorsto develop indicators which would be suitable for use in place of methylorange.- These attempts have thus far failed because the indicators developed therefrom were subject to one or more handicaps or limitations, including little or no improvementv over the methyl orange. color change; failure to accurately identify the true equivalence point of the neutralization reaction; lack of solution stability, often due to the alcoholic vehicle used; complexity of composition resulting in increased cost; and limited applicability because of incompatibility with contaminating elements.

It is accordingly an object of the presentinvention to provide a stable and economical indicator capable of wide application by virtue of its resistance to interference from contaminating elements. cator which possesses a color transition of such distinct contrast as to make the end-point unmistakably distinguishable. A further object is to provide an indicator which possesses a color transition end-point practically coincident with the equivalence point encountered in water alkalinity analysis.

The above objectives are accomplished according to the present invention by the use of solutions of the sodium salt of dimethylaminoazobenzene-o-carboxylic acid to which are added various range. 4.6 to-4.9. For example themcolor pro-. duced bya blue dye supplements the colors pro duced by the indicator during its transition, to give complementary shades *ofxgreento violet, which display a distinct *contrast at the endpoint of =the reaction. In this zcasethe violet color appears abruptly with-.the'reaching of the equivalence point involved. inxthe neutralization reactionemployed in .wateralkalinity analysis. This is usually a pH of about4.89for .boiler feed waters, though, as stated above, the pH. at the equivalence point mayrange from about 4.6 to

'4.9"for most water alkalinity analyses;

w; The following specific examples willcfurther illustrate 'the invention- 1 It is understoodthat'the invention is not limited-thereto since variations and substitutions. of dyestuffs or coloring" agents, and. 'of quantities of indicator and dyestuff', are possible within the-spirit of the invention. The

.various colour index numbers given below and in the appended claims refer to color. compounds bearing such numbers as described in Colour In- Another object is to provide an indidex, (1924), Society of Dyers and Colourists, Yorkshire, England.

Example I benzene-o-carboxylic acid is dissolved in about 300 cc. of distilled water. To this solution is added 0.50 gram of a. blue dyestuff (Colour Index No. 707) and the mixture stirred to effect solution of the dye. Sufficient distilled water is then addv edto the solution to make a total volume of.1000

Example II I 0.50 gram of sodium salt of dimethylaminoazobenzene-o-carboxylic acid is dissolved in about 300 cc. ofdistilledwater. To this :solution'is added 0.40 gram of abluedyestufi (Colour'Index No.

833), and the mixture stirred to effect solution of the dye. Sufficient distilled water is then added 7 to the solution to make a total volume of 1000 cc.

Example IV methylaminoazobenzene-ocarboxylic; acid and: a :0 water soluble blue dyestuli the ratio ofthe dyestuff to the sodium salt in saidmixture being f 0.45 gram of sodium salt of dimethylaminoazo- U benzene-ocarboxylic acid is dissolved in-zabout 300 cc. of distilled-water. To this solution is add ed0.65 gram of a blue dyestufi (Colour I'ndex No. 518), and the mixture stirred to efiect solution oi. the dye. .Sufiicient distilled 'wa'teris then add ed to the solution to make a total volume of 1000 In applying the indicator solutions to the determination of -.water alkalinity, the usual practice is'xto aidd three drops of the indicator toeach 100 :cc. of water being tested. Titrations areus- 'ually conducted at room temperatures (170-180? 'B..-):,-:although accuracy of the titration is notimaterially wafiected .by the usual variations from roomfltemperature. In alkaline solutions, the colorzproduced by the indicator solutions is green. Asthegtitration progresses the green color disappears and the solution :becomes either light-gray or colorless. This change provides the operator with :adequate warning-of the approaching endpoint. Uponxthe :completioniof the neutralization, the indicator imparts .a violet hue. to the liquid, which color is taken as the end-point for I the titration.

i-range of 4.6 to 4.9. t V

2.. indicator, characterized by the ability to Within the scope of the appended claims, var.

ious modifications and changes in thenature and proportions of the'ingredients may be made by those skilled in the art without departing from the nature of the invention.

I claim:

' .1. An indicator, characterizedby the ability to indicate the end point of'a neutralizing reaction in the determination of the alkalinity in water, consisting of a mixture of the sodium salt of di-' such as to indicate the said end point at a pH indicate the end point of a neutralizing reaction in the determination of the alkalinity in'water 1 consisting of a mixture of the sodium salt of di- Laboratory check tests'ihave ascertained that I this end-point :distinctly identifies the-equivalence point for the neutralization reaction inolved in water alkalinity analysis with a high degree .of accuracy. The efiect of interfering elements .such as phosphates, neutral salts and or- .ganic matter on indicator accuracy has also been fch'eckedand found to be negligible. Service tests "haveproven the solutions to be stable for periods greater thanone year. 'fHolwever, in the light of this information, it is not unreasonable to beiieVe-that-the solutions possess indefinite stability under normalc'onditions.

The'invention described herein may bemanu- :factured and 'used by or .for the'fiovernment of the United States of America for governmental purposes without the payment of any royalties thereon .or therefor. V

methylaminoazobenzene'fo-carboxylic acid and a water soluble r-dyestuff bearing Color lndexNl-lmher 518, the ratio of the dyestufi to the sodium salt in said mixtur .being such as .to indicate "tlj e said end pointat apH range of 4.6 to 4.9.

. 3. An indicator, characterized by the ability to consisting of a mixture of the sodium salt vof dimethylaminoazobenzene-.o carboxylic. acid and a water soluble dyestuif bearing Color Index iNum. her 714, the ratio of .the'dyestufi to the sodium salt in saidmixture being such as to indicate the H said end point atapHrangeof 4.6110 4.9,

i 7 (REFERENCES CITED Q "The :following references are of record in-the file 10f this patent: I 1 a Johnson, et al.-Ind;-Eng.-'Che'm. Anal Ea, "v01.

II, .pp. 2-4 (1930), abstracted in vo1.-24 (1930) .p. 1052. v.

Kocsis-'A.cta Lit. :Sci. Regiae Univ l-Iung. Francisco, vol. 4, ,pp. 236-8, bstracted in Chem.

Chem. Abstracts,

Abstracts, v0.1. 30 (.1935) p. 7059. 

